The present invention relates to ventilated rotors for generators, and is particularly concerned with structures for creating ventilation paths in such rotors.
Generator rotors are provided with windings to which a current is applied in order to create the magnetic flux required in order to generate output power. Typically, the windings are constituted by copper straps which are installed in slots formed in the rotor circumference, or winding face, the straps being connected together at their ends to form the windings. The flow of current through the windings results in the generation of heat which must be dissipated. Efficient heat dissipation enables higher current levels to be carried by windings in a generator having a given size.
According to one known technique for effecting such heat dissipation, cooling gas is caused to flow axially beneath each stack of conductive straps, and is then directed radially outwardly via spaced radial vents formed in the straps, as well as in insulators disposed between the straps. This is known as radial path rotor ventilation.
For a given current level, the rate at which heat evolves decreases as the effective current flow cross section of the copper conductors increases. Conversely, the larger the effective cross sections of the various vent passages, the better the cooling. Thus, for a given rotor slot cross-sectional area, the desirability of installing copper conductors having a large cross section in the direction of current flow conflicts with the desirability of providing as large a cross section as possible for the flow of cooling gas.
Various arrangements known in the art seek to resolve these conflicting considerations with varying degrees of success. One known arrangement is illustrated in FIG. 1 and includes a slot cell, or liner, 2 of insulating material which is configured to fit snugly in an axially extending rotor slot with a bottom portion 3 which rests against the slot bottom, and which extends axially along the entire length of that slot. Within cell 2 there is disposed a channel 4 of copper which is formed by bending a flat strip into approximately a C-shape, so that the upper surface of channel 4 is open along its entire length. Upon channel 4 there is disposed a stack composed of copper straps 6 and 8, with an insulating strip 10 being interposed between successive copper straps. In practice, a larger number of copper straps 8 and insulating strips 10 than illustrated will be provided in each rotor slot.
Copper strap 6 rests directly on channel 4 so that channel 4 and strap 6 act together as a single conductor. Straps 6 and 8 and strip 10 are provided with mutually aligned radial vent passages 12 for radially conducting cooling gas which initially flows axially along the passage defined by channel 4.
The structure shown in FIG. 1 requires that strap 6 be relatively thick to assure that it does not buckle during rotor assembly. Since each conductive strap 6, 8 will carry the same current level, it is desirable for all of the conductive straps to have the same current flow cross section and heat generation will be minimized by making those cross sections as large as possible. However, for a rotor slot having a given size, the larger the conductive straps, the smaller the gas flow passage provided by channel 4. Therefore, in order for the passage provided by channel 4 to have the necessary cross-sectional area, strap 8, as well as the straps thereabove, must be made thinner than strap 6. As a result, the rotor will operate at higher temperatures and will require greater input power.
A second prior art approach is illustrated in FIG. 2. Here, each rotor slot has a channel 14 milled in its bottom to provide an axial flow path for cooling gas. Upon channel 14 there is placed a relatively thick insulating creep spacer 16 which supports slot cell 18, of insulating material, and a stack composed of copper straps 20 and interposed insulating strips 22. Here again, the number of straps 20 and strips 22 provided in practice will be greater than that illustrated in FIG. 2. All straps 20 and strips 22 are provided with mutually aligned vent passages 24 for radially conducting cooling gas which initially flows axially through channel 14.
The structure illustrated in FIG. 2 includes a considerable amount of insulation, thereby reducing the space available for conductive straps. Moreover, there is a considerable danger, with this structure, that the high velocity cooling gas traveling axially through channel 14 will cause flaking of material from spacer 16, resulting in the danger of blockage of the radial vents, which will cause thermal imbalances. Moreover, in order to create the required radial vent passages, holes must be punched in the bottom of slot cell 18, and this compromises the structural integrity of that component.